Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay, after purification and beneficiation, is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The whiteness or brightness makes kaolin useful in applications such as coatings and fillers in paper and paint. However, kaolin clays also contain a variety of discoloring titanium and iron phases, two of these being anatase (TiO.sub.2) and iron oxides, which detrimentally affect the brightness of kaolin. Therefore, there is a need to remove the discoloring phases, thereby improving the brightness and making the clay acceptable for pigment applications. Some mineral removal processes used in the beneficiation of kaolin include flotation, magnetic separation and selective flocculation.
Flocculation involves the aggregation of fine particles which are suspended (i.e., dispersed) in liquid by a bonding agent (i.e., a flocculant) that attaches to the particles. In general, the flocculant is initially adsorbed on the particles and bonds adjoining particles. The bonded particles then form larger aggregates or flocs which settle or sediment out of the suspending liquid. Flocculants can be natural products such as starch, guar gum and alginates or synthetic polymers such as polyacrylamides, polyacrylates and polyethylene oxides.
In a selective flocculation process, the objective is the selective removal of certain minerals from a mixture of mineral particles. To achieve this selectivity, the flocculant should only adsorb on certain types of mineral particles.
Selective flocculation is an effective process for recovering fine to ultrafine minerals that respond poorly to conventional beneficiation techniques. The successful utilization of selective flocculation on mixtures of fine mineral particles such as kaolin clays, iron-bearing minerals, phosphates, potash, copper-bearing minerals and coal is known in the industry.
In the selective flocculation of kaolin, the removal of titanium and iron minerals may be accomplished by flocculating the kaolin clay and leaving the dispersed discolored impurities in suspension or vice versa (i.e., the impurities are flocculated and the clay is left in suspension). The flocculated material is then settled while the dispersed phases are decanted or siphoned off to obtain the desired product.
In a first type of selective flocculation, U.S. Pat. No. 3,808,021 describes a method of selective flocculation where the kaolin is flocculated and the impurities are left dispersed in solution. This process utilizes a strongly anionic polyacrylamide polymer having a molecular weight in excess of 1,000,000. In general, the process uses phosphate-based dispersants (e.g., tetrasodium pyrophosphate and sodium tripolyphosphate) to disperse the kaolin and then flocculates the kaolinite with an anionic flocculant, which causes the kaolinite to settle and separate from the titanium impurities which remain in suspension. The clay flocs are then redispersed by high shear mixing, followed by multiple washings and delamination.
U.S. Pat. No. 3,837,482 flocculates the clay by dispersing the kaolin slurry at highly alkaline conditions with an alkali metal hydroxide and a polyanionic clay dispersant (preferably alkali metal silicate) and then flocculating with a weakly anionic partially hydrolyzed polyacrylamide. The clay then settles, leaving the discolored titanium and iron minerals in suspension.
In a second type of selective flocculation of kaolin, the iron and titanium mineral phases are flocculated and settled instead of the clay. In general, this method selectively activates the titanium and iron phases within a dispersed kaolin slurry, followed by flocculation of the discolored phases with a polymeric flocculant which causes such phases to sediment and separate from the clay.
U.S. Pat. No. 4,604,369 discloses a selective flocculation process involving the dispersion of kaolin clay using conventional dispersants and an ammonium salt conditioning agent which activates the titanium-containing phases. Activation is followed by the addition of a weakly anionic high molecular weight polymer which selectively flocculates the titanium minerals which are then removed by sedimentation.
U.S. Pat. No. 3,701,417 utilizes soluble salts as activators. U.S. Pat. No. 3,862,027 utilizes polyvalent cations (e.g., calcium, magnesium and barium) to condition the titanium-bearing phases before flocculating these phases with an anionic polymer.
U.S. Pat. No. 3,826,365 selectively flocculates the titanium and iron minerals using polyvalent cations. This method removes the flocculated phases by magnetic separation.
U.S. Pat. No. 3,857,781 flocculates the titanium and iron phases by first overdispersing the kaolin using dispersants (e.g., sodium hexamataphosphate and sodium silicate) followed by treatment of the flocculated slurry with sodium chloride. After aging, the slurry is mixed with a high molecular weight strongly anionic polyacrylamide which selectively flocculates the iron and titanium phases, thereby separating these phases from the clay.
U.S. Pat. Nos. 5,535,890; 5,584,394; and 5,603,411 disclose the use of polyvalent cations (e.g., calcium chloride) and fatty acids as activators for the iron and titanium phases. After the slurry is preconditioned with these activators, selective flocculation of the discoloring impurities is achieved by using a high molecular weight organic polymer (e.g., highly anionic polyacrylamide and/or copolymers of acrylate and acrylamide). These patents disclose that this process produces a highly purified concentrate of kaolin and a clay fraction concentrated with titanium and iron.
The first type of selective flocculation of kaolin is not practical since the reagent consumption is high because the majority of the kaolin must be flocculated. Also, contamination of the clay product by the flocculant occurs, requiring additional processes to remove the flocculant. The second method is more practical, but good selectivity of the flocculant adsorption is critical.
As shown in the above patents related to selective flocculation of iron and titanium minerals, activation of the discoloring impurities can be achieved using various monovalent, divalent and trivalent cations. The mechanism with which the cations adsorb and activate the various phases is electrostatic attraction; that is, interaction of oppositely charged species. To specifically activate the titanium and iron phases, the chemical environment (i.e., pH, dispersants present) has to be such that adsorption is specific for the discoloring minerals to be flocculated. However, even when the chemical environment is optimal, the selectivity of the various cations for the mineral surfaces can be low. The use of a fatty acid with the polyvalent cation may improve selectivity. However, if the selectivity is not high, the results can be poor mineral rejection and low clay recovery. Thus, the selectivity of the flocculation process for titanium and iron minerals is of utmost importance in optimizing the process and increasing clay recovery.
Selective flocculation is also being used to treat other mixtures of mineral particles. Iron-bearing ores, specifically taconite, are being commercially processed using selective flocculation. The fine ore is ground with caustic soda and sodium silicate to disperse the mineral particles. The ground ore is then flocculated with a corn-starch flocculant to selectively remove hematite, an iron oxide mineral. Xanthate-containing polymers were used to selectively flocculate copper- (e.g., chalcopyrite and chrysocolla) and lead- (e.g., galena) containing minerals (Krishnan & Attia, Reagents in Mineral Technology, Chapter 16, pp. 508-509, Marcel Dekker Inc., 1988). Separation of phosphate minerals from associated clays by selective flocculation was carried out using sodium silicate as a dispersant and starch as a flocculant (U.S. Pat. No. 2,660,303).
Potash has also been reported to be processed by selective flocculation using a nonionic polyacrylamide flocculant and/or ethoxylated alkylamic alkylguanidine complex (Krishnan & Attia, Reagents in Mineral Technology, Chapter 16, p. 510, Marcel Dekker Inc., 1988). U.S. Pat. No. 5,535,890, which discloses a selective flocculation process that uses fatty acid and polyvalent cations to precondition the mineral suspension, also claims application of the process to beneficiating alkaline carbonate minerals, phosphate minerals, zeolites and bauxites.
The literature discloses that the selectivity of the flocculating agent can be enhanced by the introduction of chelating functional groups. Clauss, Appleton & Vink (International Journal of Mineral Processing, Vol. 3, pp. 27-34, 1976) describe selective flocculation of cassiterite using a modified polyacrylamide flocculant which contains hydroxamate functional groups. Ravishankar, Pradip, Deo, Kulkarni & Gundiah (Bulletin of Material Science, Vol. 10, No. 5, pp. 423-433, 1988) use a modified polyacrylamide containing up to 8.3% hydroxamate functional groups to selectively flocculate iron oxide from kaolin. In this latter work, an artificial mixture of kaolin and synthetic iron oxide was dispersed with sodium silicate at a pulp density of 1%; the hydroxamate-containing modified polyacrylamide was then added to flocculate and sediment the iron oxide.
The selectivity of chelating agents for certain minerals is well known in flotation processes. Alkyl or alkaryl hydroxamic acids have shown good selectivity towards minerals containing Ti, Y, La, Ce, Nb, Ta, Sn, Fe, Mn and Cu (Nagaraj, Reagents in Mineral Technology, Chapter 9, pp. 289-296, Marcel Dekker Inc., 1988). Hydroxamates are powerful collectors in flotation due to this specificity for a variety of metals with which the hydroxamate can chelate. A variety of minerals containing these metals have been successfully floated as discussed in U.S. Pat. No. 3,438,494 which describes the flotation of chrysocolla (a copper-bearing silicate mineral) and iron oxides. U.S. Pat. Nos. 4,629,556 and 5,522,986 describe the use of hydroxamates in the flotation of titanium and iron phases from kaolin clays. However, hydroxamates have not been described in the selective flocculation of kaolin clays.
Therefore, there is a need in the industry for an effective selective flocculation process to remove certain impurities from kaolin clays and other mixtures of mineral particles.